Open Warcraft 3 -> Near the "Battle.net" button, there is an other one, click it -> select EUROBATTLE (and click "OK" -> Click on "Batttle.net" -> enter with your account and password (and click "Logon") -> Click "Enter Chat" -> on the bottom of the screen, there is a box, you can type in it. Type "/join playdota.eu" -> you are in PlayDotA.eu's channel -> In the same box use the commands and play.
I (and I think noboby else) can say it more clearly.